1. Technical Field
This invention relates to coordinate measuring machines and more particularly to an improved and simplified pneumatic counterbalance for offsetting the weight of the vertical probe arm used on a coordinate measuring machine.
2. Background Art
Coordinate measuring machines are well known in the prior art. In a common type of coordinate measuring machine, which is used to make measurements of an article, a probe is mounted at the end of a vertically movable probe arm. The vertically movable probe arm is typically supported by a cartridge which is horizontally movable. Movement of the probe arm or carriage is accomplished in a manual type machine by an operator grasping the probe and physically moving it to the desired position, or, in a more sophisticated computer controlled machine, by a motor and a controller automatically positioning the probe.
In order to measure the article, the probe and carriage must be easily movable. Ideally, the probe, the carriage, and the related apparatus should be light weight. The vertically movable probe arm should be easily movable in response to desired positioning by an operator. However, the probe and the associated probe arm must not be movable freely or when no movement is desired.
It is also necessary that a coordinate measuring machine must be very accurate. Such accuracy is dependent upon low friction which in turn is related to the weight of the movable apparatus which is supported from the base of the coordinate measuring machine.
Several systems have been suggested to counterbalance the weight of the probe arm shaft assembly to prevent it from falling down and thereby possibly causing injury or damage. Prior art counterbalancing systems include mechanical counterbalances, pneumatic counterbalances and electrical counterbalances.
One method of accomplishing mechanical counterbalancing is to provide the probe shaft with an equivalent weight mounted to and offsetting the weight of the probe assembly. A disadvantage of such an arrangement is that the equivalent mechanical weight adds significantly to the friction and inertia in the system and makes it more difficult to move the probe and the carriage to accomplish the measurements on the article.
Another mechanical counterbalance approach, used in the prior art, couples the probe to a spring to provide an equivalent force directly opposite to the weight of the probe assembly. Unfortunately, springs are not available which exert a uniform force over a typical operating range of probe shaft movement, which is normally between 8 and 40 inches depending on the size of the machine. Further, the use of a spring counterbalance could result in friction which is greater than desired in some applications.
An electric counterbalance is disclosed in U.S. Pat. No. 3,818,596. In this patent, a counterbalance arrangement is disclosed which uses a variable torque magnetic particle clutch which has an output which is varied in accordance with the weight of the probe shaft.
Prior art counterbalances have typically had more friction than desired which makes it difficult to move the probe shaft. Further most prior art systems have characteristically been large and heavy, which have an unsatisfactory effect on the accuracy and repeatability of the coordinate measuring machine.
U.S. Pat. No. 4,207,680 which is herein incorporated by reference, discloses a pneumatic counterbalance system which has been found to operate satisfactorily.